How Many Layers To Stack W/o Cardboard For A Carved Cake?

One of the popular cakes today shows a rockwall climber. The cake is obviously tall and narrow, but even if it wasn't I know that some cakes just need a lot of layers to get the desired effect. I've tried to figure out what Duff does on Ace of Cakes, and it seems that four is the normal max, but then I've seen them carving cakes with more layers than that and they don't seem to be running into cardboard. My attempts at whimsical and tall, carved cakes have been less than graceful, and this is something I really want to accomplish. Help!?!?

Every 4 inches is the standard. When carving on one episode, Geoff showed that he tries to match the size of the board to where he's going to cut. (He actually hit a small slice of one on that episode.)